Neurodevelopment is dependent on the action of thyroid hormones, and disruption of this hormonal axis during sensitive periods of development could increase the risk of clinical disorders like attention- deficit/hyperactivity disorder or learning disabilities. Phthalate, triclosan, and bisphenol A (BPA) exposures are nearly universal in pregnant women and children, and could adversely affect thyroid hormones and brain development. No studies have prospectively linked these exposures to both thyroid hormones and neurodevelopment to determine if and when the developing fetal or child brain is more sensitive to these early life exposures. We will address this gap using an existing prospective cohort of 356 mother-child pairs who have been followed from the 2nd trimester of pregnancy until their child is ~8 years old. We will comprehensively assess pre- and postnatal phthalate, triclosan, and BPA exposure using infant meconium samples, two pregnancy urine samples, and up to six urine samples from infancy/childhood. Child cognitive abilities and behavior have been repeatedly assessed from 1-8 years of age and thyroid hormone levels will be measured during pregnancy, at birth, and 1, 3, 5, and ~8 years of age. These data will be systematically investigated to: 1) determine if and when early life phthalate, triclosan, and BPA exposures have an adverse effect on children's cognitive abilities and behavior between 1-8 years of age and 2) investigate the association between early life phthalate, triclosan, and BPA exposure and maternal/child thyroid hormone concentrations to determine if thyroid hormones mediate the associations between chemical exposures and neurobehavioral outcomes. Innovative features of this project include detailed prospective exposure assessment using meconium biomarkers to directly assess fetal exposure and up to 8 urine samples during gestation and childhood, repeated neurobehavioral assessments, hormonal intermediates that are essential for neurodevelopment and potentially sensitive to these chemical exposures, and sophisticated statistical methods to pinpoint the period of maximum sensitivity to these chemicals. These data will inform public health agencies about the toxicity of highly prevalent and potentially modifiabl environmental chemical exposures and help target effective public health interventions aimed at reducing exposure during the most sensitive periods of brain development.